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Topic: Assigning pin values (Read 854 times) previous topic - next topic

ssande

While looking through sample code examples for assign pin values to a variable I see that people are using integers like:
Code: [Select]
int ledPin = 13;                // LED connected to digital pin 13
Is there a reason for not defining the variable as a byte since that is adequate to hold values 0-255?

Scott

CrossRoads

I use byte. Others use #define, #const, to keep the software from changing the pin assisgnment accidentally I suppose.
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AWOL

int is the natural unit to think in, but I think it gets cast down to byte for calls to digitalWrite anyway
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ssande

Thanks all.  I am coming from the Basic Stamp and out of necessity you did not waste any memory or variables.  Really starting to like the Arduino!

dgerman

#4
Mar 30, 2012, 07:54 pm Last Edit: Apr 11, 2012, 12:54 am by dgerman Reason: 1

            • LED_BUILTIN is a defined symbol in pins_arduino.h. It should be used. Don't made a variable for it.

            • (as of 1.00 4/10/12 it is incorrectly defined if using ETHERNET board, it should be 9 )



            • There is a big difference between using #define and int
              • #define
                acts as an alias and is a preprocessor directive. The substitution occurs when the source is compiled. It allocates no storage. The hardware instruction set contain a class of short "immediate" instructions which do not require fetching an operand for small values, like 13. Using define permits the compiler to use them

              • int
                • allocates 2 bytes in precious RAM (even byte takes one).

                • In addition the generated code cannot use an immediate instruction.






            • The value for LED_BUILTIN is not actually the number of a hardware pin, rather is is a software definition used by the library functions
            • Unfortunately digitalWrite  as defined in wiring_digital.c as seen in some examples is hugely  more complicated then simply setting a value on a pin. The code generated for the call is longer with the int
              • with define:
                • 8d e0           ldi r24, 0x0D   ; 13
                • 61 e0           ldi r22, 0x01   ; 1
                • 0e 94 41 01     call    0x282   ;  0x282

              • with int
                • 80 91 00 01     lds r24, 0x0100 ;  LED13

                • 61 e0           ldi r22, 0x01   ; 1

                • 0e 94 61 01     call    0x2c2   ;  0x2c2



            The hardware instruction set for the ATmel AVR micro contoller (as well an many other micro computer systems ) includes other immediate instructions. These include instructions for setting, resetting and testing flags. They handle I/O ports extremely efficiently. There also exist immediate instructions to  add, subtract(with and without carry), and, or and compare as well as skip instruction which test I/O ports directly. They require referencing the specific port and pin.
            Refer to ATmel AVR instruction set document 0856I-AVR-07/10 at
http://www.atmel.com/Images/doc0856.pdf

For example  CBI Clears a bit in an I/O register which is specified in the instruction takes only 2 bytes (see page 48)

    ++++++++++++++++++++++++++
Can the reference pages
http://arduino.cc/en/Reference/Int and  
http://arduino.cc/en/Reference/UnsignedInt and
http://arduino.cc/en/Reference/DigitalWrite
http://arduino.cc/en/Reference/PinMode
please be revised to replace unsigned int ledPin = 13;  with something better.


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